PROJECT SUMMARY: In animal cells, cytokinesis is driven by constriction of an actomyosin contractile ring, which is positioned and controlled by signaling from spindle microtubules. It has long been assumed that all animal cells divide by a similar molecular mechanism. Yet variation in cytokinesis, or diversity in mechanistic and regulatory pathways, is becoming increasingly clear for different cell types within multicellular organisms. The mechanisms underlying this cell type-specific regulation of cytokinesis remain poorly understood. In preliminary studies, we established the C. elegans 4-cell embryo as a system to study cell type-specific regulation of cytokinesis. At the 4-cell stage, each individual cell already has a unique cell fate, specified by conserved cell fate signaling pathways (e.g. Src, Wnt/frizzled, Notch/Delta) and dependent on direct cell-cell contact between specific cell pairs within the embryo. Using either chemical (LatrunculinA) or genetic (formin temperature sensitive mutant) perturbations to weaken the F-actin contractile ring, we identified cell type- specific regulation of cytokinesis in the 4-cell embryo. We found that two of the four cells are more protected against cytokinetic stress than the others and can divide successfully when the F-actin cytoskeleton is weakened. Embryo micro-dissection and cell pairing experiments revealed that in one of these two protected cells (P2), cytokinetic protection is cell-intrinsic and dependent on germline fate specification, whereas in the other protected cell (EMS), cytokinetic protection is cell-extrinsically regulated and requires direct cell-cell contact with its neighbor cell (P2) and Src-dependent cell-fate signaling. Using our collection of ts mutants, we also identified cell type-specific protection against cytokinesis failure upon damage to the spindle signaling machinery in both the 4- and 8-cell embryo. The experiments in this proposal will determine the molecular mechanisms by which cell fate specification regulates cell type-specific variation in cytokinesis in three ways: 1) we will identify the cellular and molecular mechanisms that underlie cell fate-dependent cytokinetic protection after actin-based damage; 2) we will compare the cell type-specific protection against cytokinesis failure after spindle signaling-damage to that after actin-damage in the 4- thru 16-cell embryo and identify conserved and cell type-specific mechanisms that protect against different cytokinetic stresses; and 3) we will determine if cell fate specification can also protect cytokinesis from stress in the context of a multicellular tissue, as well as test whether the principles that apply to the early embryo also apply to mammalian cells in culture. The proposed experiments will define the role of cell fate signaling in promoting variation in cytokinesis, identify specific cell fate pathways and test their mechanisms of action, and determine the universality of cell type-specific variation in cytokinesis. Because both cytokinesis failure and dysregulation of cell fate signaling are both emerging as biomarkers for human diseases such as cancer and are regulated by evolutionarily conserved molecular mechanisms, our work will have relevance for human health.